Charging an electric furnace

ABSTRACT

A metallic charge is introduced into an electric furnace in a continuous fashion using as a feeding device an unobstructed tubular container of undulant or zig-zag shape. The feeding device is oriented horizontally and is rotated about its longitudinal axis causing the charge to advance through the container and discharge into the furnace through a roof port. The feeding device can handle a variety of material sizes and shapes, ranging from powder to shredded scrap, singly or in admixture, without modification. Hot furnace gases are vented through the feeding device thus cooling the gases and preheating the furnace charge.

Tress et al.

1 CHARGING AN ELECTRIC FURNACE [75] Inventors: Jack E. Tress; Willard 1. Hunter,

both of Albany. Orcg.

[73] Assignee The United States of America as represented by the Secretary of the Interior, Washington. DC.

[22] Filed: Aug. 26, 1974 1211 Appl. No: 500,797

Related US. Application Data [62] Division of Ser. No. 4501136. March 13 1974.

152] US. Cl 13/33; 198/1; 214/18 R 151 1 Int. Cl. F27D 3/08 [58] Field ofSearch 13/10. 33;214/l8 R, 18 K; 75/11; 266/27 [56] References Cited UNITED STATES PATENTS Z 44h (137 8/1948 Cramptun et ul. 13/33 X 3.163.521) 12/1964 Collin ct tll. 13/33 X 1 Aug. 19, 1975 11/1968 Dillon et a1. 13/10 X H1974 Brusa 13/33 X Primary Exuminer-R. N. Envall, Jr. Attorney. Agent, or Firm-Roland H. Shubert; Donald R. Fraser 5 7 1 ABSTRACT The feeding device can handle a variety of material sizes and shapes, ranging from powder to shredded scrap, singly or in admixture, without modification. Hot furnace gases are vented through the feeding device thus cooling the gases and preheating the furnace charge.

8 Claims, 2 Drawing Figures DPT" nut CHARGING AN ELECTRIC FURNACE This is a division of application Ser. No. 450,636, filed Mar. 13. 1974.

BACKGROUND OF THE INVENTION This invention relates broadly to the processing of metals in an electric furnace. A specific embodiment of this invention relates to the continuous charging of ferrous metal materials of differing sizes and shapes into an electric furnace in a continuous fashion for the purpose of melting or refining. The ferrous materials may be in the form of iron powder, sponge iron briquettes, prereduced iron ore pellets or shredded scrap metal such as that obtained by the Proler method. These ferrous materials may be continuously fed either singly or in admixture into an electric furnace in counterflow relationship to hot furnace gases to thereby cool the gases and preheat the furnace charge.

With the advent of the basic oxygen furnace and the development of prereduced iron ore pellets, the use of the electric furnace in steel making has obtained greater importance. The proportion of scrap iron in the charge to a basic oxygen furnace is severely limited while an electric furnace can operate on charges of essentially I percent scrap. Highly metallilzed pre reduced iron ore pellets constitute a desirable feed for an electric furnace because of the ease of which the pellets can be transported, stored and charged into the furnace and because of their uniformity in composition.

The majority of electric furnaces today are top charged. That means that the charge is dumped into the furnace after the roof has been removed or swung away. The furnace charge is placed in a drop-bottom bucket and released into the furnace. This method of charging results in a very quick loading of the furnace but. because of the bulk of scrap metal, it is usually necessary to charge the furnace two or more times for a single heat. Removal of the roof from a furnace containing molten metal results in rapid cooling of the furnace and roof linings and considerable heat loss by radiation. Such rapid temperature changes may lead to spalling and other damage to the refractory linings but this damage is minimized by holding the charging time to a short a period as possible. In addition to heat loss this method of furnace charging tends to decrease production because of the furnace dead time during the charging procedure.

Gases and fumes produced in the furnace are usually vented through a port in the roof and treated to remove particulate matter. Gases vented from the furnace represent a loss in sensible heat as their exit temperature is usually in the range of about l0()() to I200C with a ferrous metal furnace charge. Additionally, these exhaust gases represent a loss of chemical energy proportionate to the amount of carbon monoxide and other combustible gases they contain. The gases must be cooled prior to the removal of particulates by such means as electrostatic precipitators or baghouses and this heat energy carried by the gases is seldom recovered.

There have been attempts to deve op techniques for the continuous feeding of ferrous materials into electric furnaces. Techniques and devices now available operate well on a homogenous charge material. Prereduced iron ore pellets in particular are amenable to continuous charging because of their flow characteris- LII tics and their uniformity in size and shape. However, up to this time techniques and devices for the continuous charging of materials into an electric furnace lack versatility; they are tailored for a specific type and size of charge material.

SUMMARY OF THE INVENTION Metallic materials ranging in size and shape from powders to shredded scrap, either singly or admixture, may be continuously charged into an electric furnace using a tubular container of undulant shape rotating about its longitudinal axis as a feeding device. The feeding device preferably discharges into a conduit communicating with the interior of the furnace through the furnace roof. Furnace gases are vented through that same conduit and pass in counterflow fashion through the feeding device. The tumbling action imparted to the charge by the feeding device insures excellent contact between the entering charge and the exiting gases thus preheating the charge and cooling the gases. In a preferred embodiment. air in sufficient quantity to burn up all carbon monoxide contained in the exiting furnace gases is allowed to enter the system at the juncture between the rotating feeder and the stationary roof conduit. This simplifies the system in that problems of providing a seal exposed to high temperature gases are alleviated. It also substantially reduces the carbon monoxide content of the furnace gases while recovering much of the heat energy released by the combustion of carbon monoxide. Gas flow through the feeding device is controlled by means of an induced draft fan located downstream to the gas flow from the feeding device.

Hence, it is object of our invention to provide a method and means for continuously feeding a variety of particulate metallic materials into an electric fur nace.

It is another object of our invention to recover the heat energy from vented furnace gases to preheat the furnace charge while cooling the gases so that they be further cleaned before venting to the atmosphere.

DETAILED DESCRIPTION OF THE INVENTION The invention will be more particularly described with reference to the illustrated embodiments thereof shown in the accompanying drawings wherein:

FIG. 1 is a schematic partial sectional view of a plant utilizing one preferred form of apparatus for carrying out the process of this invention.

FIG. 2 illustrates details of the feeding device used in the apparatus of FIG. I.

Use of our invention provides the following advan tages and benefits as compared to the conventional practices now employed:

1 A variety of metallic feed materials can be charged to an electric furnace in a continuous fashion without modifying, adjusting or changing the feeding apparatus.

2. Furnace dead time is reduced because operation of the furnace need not be halted for charging.

3. Savings in heat and energy are realized because the furnace roof is not removed for charging and heat content of the furnace gases is utilized to preheat the charge.

4. Thermal stresses on the refractory lining of the furnace side walls and roof are substantially reduced because the furnace need not be opened up for charging.

Referring now to FIG. I. the particulate metallic material to be charged into the electric furnace is contained in hopper or bin 1 and is introduced into rotating feeder 2 by means of control apparatus 3 and chute 4. Feeder 2 comprises a container 5 fabricated of metal capable of withstanding the hot gases emanating from an electric furnace. Container 5 is formed of a series of intercommunicating truncated tubular sections 6 which form a series of V-shaped members disposed in a zigzag relation in one plane. Container 5 is oriented with its longitudinal axis in a substantially horizontal attitude and is mounted upon base number 7 for rotation about its longitudinal axis. Trunnion wheels 8 carried by brackets 9, which are mounted upon base 7, engage corresponding trunnion rims 10 to support the container for rotation. Container 5 terminates in cylindrical end members 14 and 15. A rotational force is applied to container 5 in any suitable fashion such as by means of motor 11 driving container 5 through a chain or belt 12 and a sprocket or pulley means 13 mounted on end member 14. End member 15 is rotatably disposed within hub 16 which is mounted on stationary conduit means 17. Conduit means 17 in turn communicates with the interior of electric furnace 18 by way of roof port 19.

Electric furnace 18 is of conventional construction being provided with a refractory lining 20 and a removable roof 2]. Heat is supplied to the charge within the furnace by means of current carrying electrodes 22 which pass through the furnace roof. Liquid metal 23 and slag 24 collect in the bottom of the furnace and may be periodically withdrawn through tap holes (not shown) in the side of the furnace as is known in the art. Alternatively, the furnace may be tapped by tipping the contents of the furnace into a ladle as is also conventional. carbonaceous reducing agents such as coke or slagging agents such as limestone of silica may be supplied to the furnace from hopper 25 through control device 26 and conduit 27 which communicates with the interior of the electric furnace by way of roof port 28. Alternatively, slagging agents may be introduced into the furnace by way of the feeder-preheater in association with the furnace charge.

Particulate metallic materials to be melted or refined are passed from bin 1 into cylindrical end section 14 of container 5. Rotation of the container causes the material to advance through the feeder and be discharged into chute l7 and thence into the furnace. Gases produced by operation of the furnace are vented through roof port 19 and pass in counterflow fashion to the incoming charge material through chute l7 and container 5. The tumbling action imparted to the charge by rotation of container 5 insures excellent contact between the exiting furnace gases and the charge material. Cooled gases are drawn from container 5 by way of hub 29 and conduit 30. Seal means 31 provides a juncture between rotating end member 14 of container 5 and stationary hub 29. Furnace gases. cooled by passage through container 5, are subjected to a preliminary cleaning step in cyclone 32 by action of induced draft fan 33 which communicates with the cyclone discharge by way of conduit 34. Cleaned gases may be dis charged from fan 33 directly to the atmosphere or, in appropriate cases, the gases may be subjected to fur ther cleaning as in a baghouse or electrostatic precipitator. Particulate materials separated from the gases may be removed from the cyclone by way of conduit 35. Induced draft fan 33 may be located ahead of cyclone 32 with equivalent result.

ln steel making operations. the gases vented from an electric furnace are at a temperature within the range of [000 to l200C and often contain significant quantities of carbon monoxide. It is preferred, but not required, that hub 16, which connects cylindrical end portion 15 and stationary chute 17, be constructed so as to allow air leakage into the system at this point. Such deliberate air leakage alleviates many problems with rotating seals operating at these temperatures and provides a source of oxygen to burn up carbon monoxide and any other combustible materials which are contained in the furnace vent gases. Much of the heat released by the combustion reaction is utilized in preheating the furnace charge. In the case where iron powder comprises the charge material, care must be taken to limit gas flow through the feeding device to a rate such that iron particles are not entrained by the gas stream.

Referring now to FIG. 2, feeder 5 is shown in greater detail. Truncated tubular sections 6 are preferably joined at an angle of to l35 to each other to form a series of interconnected V-shaped chamgers. It is preferred that each tubular section be set at an angle of 45 to the longitudinal axis, or axis of rotation, of the device. The number of tubular sections making up the feeder may vary but it is preferred that there be provided a minimum of6 such sections. The number of angled tubular sections and the speed of rotation determine the residence time of the charge material. The following equation approximates the residence time:

4 v v Mlnimum residence time (min) Number of an lcd sections +2 rpm to provide longitudinal stability for the device. Disposed circumferentially within end member 14 is material guide means 36 which may be a ribbon-like member in the form ofa single or multiple helix. Upon rotation of the feeder, member 36 causes the feed material to advance into the zig zag portion of the device.

A similar helical member 37 is disposed circumferentially within cylindrical end section 15 at the discharge end of the feeder. With the exception of helical members 36 and 37 the interior of the feeder device is entirely unobstructed. It is preferred that the exterior of the feeding device be well insulated so as to conserve heat energy and to provide for safer operation.

In a preferred embodiment the furnace is operated in a semicontinuous fashion without removing the roof between heats. This can be accomplished by providing two tap holes (not shown in the drawings) in the furnace wall to periodically tap slag and molten metal. At the completion of a heat, the slag may be tapped from the furnace through an upper tap hole after which molten metal may be tapped through a lower tap hole. The lower tap hole is preferably located slightly above the level of the furnace floor so that a residue of molten metal remains in the furnace after tapping. Leaving a residue of molten metal within the furnace allows an immediate resumption of furnace operation without arc damage to the floor of the furnace. Alternatively, both slag and metal can be tapped from a single top hole located slightly above the level of the furnace floor. When a tilting electric furnace is used, slag and metal can be separately tapped through a single tap hole located aoove the slag line as is also well known.

Most electric furnaces utilize three electrodes arranged in delta configuration near the center of the furnace. When such an electrode configuration is used, it is preferred that the charge material be fed into the middle of the delta. It is also preferred to operate the furnace during the melt down phase of a heat with a surplus of unmelted charge stock present in the furnace. A surplus of unmelted charge stock in close proximity to the electrodes increases efficiency of the process and tends to provide some shielding of the refractory from the arc.

In some instances it is desirable to provide a further refining step for the molten metal. In such cases, slag may be tapped from an upper tap hole at the completion of the melting phase of a heat, and additional refining agents or fluxes may be added. Slag formed during the refining step may then be tapped after which the molten metal may be withdrawn as was described before.

Energy savings realized by use of the process and apparatus of this invention were determined experimentally to be on the order of percent when charging ferrous materials such as pre-reduced iron ore pellets and shredded scrap. This comparison was made using the same electric furnace charged from the top after removing the roof as opposed to a continuous introduction of charge materials. Furnace gases entering the discharge end of the feeder were at a temperature in excess of IUUUC and were cooled to a temperature less than 300C after passage through the feeder. Retention time of the charge material within the feeder was slightly in excess of one half minute. A controlled leakage of air was allowed at the juncture between the rotating feeder and the stationary chute leading to the electric furnace. The carbon monoxide content of the furnace gases after passing through the feeder was essentially zero.

We claim:

1. Apparatus for melting and refining particulate metals which comprises:

an electric furnace closed to the atmosphere;

conduit means communicating between the interior of the electric furnace and the discharge end of a rotating feeding device, said feeding device comprising an unobstructed tubular container of undulant shape disposed with its longitudinal axis in a substantially horizontal attitude and rotatable about that axis;

means to introduce particulate metal charge material into the end of said container opposite said discharge end;

means to rotate said container about its longitudinal axis to thereby advance said charge material through said container, and

means to withdraw gases produced within said electric furnace through said conduit means and through said feeding device in counterflow rela' tionship to said advancing charge material.

2. The apparatus of claim 1 wherein said means to withdraw gases produced within said electric furnace comprises an induced draft fan located downstream to the gas flow from said rotating feeding device.

3. The apparatus of claim 2 wherein said tubular container of undulant shape comprises a plurality of truncated, tubular sections forming a series of V-shaped members disposed in a zig-zag relationship in one plane.

4. The apparatus of claim 3 wherein the discharge end of said tubular container and the end of said container opposite said discharge end comprise cylindrical sections disposed with their geometrical axis in alignment with the longitudinal axis of said tubular container.

5. The apparatus of claim 4 including a sealing hub member disposed between said conduit means and said cylindrical discharge end section.

6. The apparatus of claim 5 wherein said hub member is adapted to allow a controlled gas flow from the atmosphere into said tubular container.

7. The apparatus of claim 4 wherein the electrodes of said electric furnace are three in number, are arranged in delta configuration, and are located in the central portion of said furnace.

8. The apparatus of claim 7 wherein said conduit means communicates with the interior of said electric furnace at a location in the middle of the delta formed by said electrodes. 

1. Apparatus for melting and refining particulate metals which comprises: an electric furnace closed to the atmosphere; conduit means communicating between the interior of the electric furnace and the discharge end of a rotating feeding device, said feeding device comprising an unobstructed tubular container of undulant shape disposed with its longitudinal axis in a substantially horizontal attitude and rotatable about that axis; means to introduce particulate metal charge material into the end of said container opposite said discharge end; means to rotate said container about its longitudinal axis to thereby advance said charge material through said container, and means to withdraw gases produced within said electric furnace through said conduit means and through said feeding device in counterflow relationship to said advancing charge material.
 2. The apparatus of claim 1 wherein said means to withdraw gases produced within said electric furnace comprises an induced draft fan located downstream to the gas flow from said rotating feeding device.
 3. The apparatus of claim 2 wherein said tubular container of undulant shape comprises a plurality of truncated, tubular sections forming a series of V-shaped members disposed in a zig-zag relationship in one plane.
 4. The apparatus of claim 3 wherein the discharge end of said tubular container and the end of said container opposite said discharge end comprise cylindrical sections disposed with their geometrical axis in alignment with the longitudinal axis of said tubular container.
 5. The apparatus of claim 4 including a sealing hub member disposed between said conduit means and said cylindrical discharge end section.
 6. The apparatus of claim 5 wherein said hub member is adapted to allow a controlled gas flow from the atmosphere into said tubular container.
 7. The apparatus of claim 4 wherein the electrodes of said electric furnace are three in number, are arranged in delta configuration, and are located in the central portion of said furnace.
 8. The apparatus of claim 7 wherein said conduit means communicates with the interior of said electric furnace at a location in the middle of the delta formed by said electrodes. 